1. Field of the Invention
The present invention relates to an electroconductive composition to be used for forming electroconductive films of various electronic components and the like, and a method for forming an electroconductive film by adopting such an electroconductive composition.
2. Description of Relevant Art
In various electronic components and the like, there is formed a pattern of an electroconductive composition by using a technique such as screen printing, which pattern is calcined at a low temperature to form a thick-film electroconductive circuit having a low resistance. Typical methods for forming thick-film electroconductive circuits are generally classified into subtractive methods and additive methods, and the additive methods have advantages over the subtractive methods in terms of readiness, cost, and the like of process, and environment loads due to wastes.
Examples of characteristics typically required for electroconductive compositions to be used in additive methods include readiness of formation of electroconductive circuits, lower specific resistances of electroconductive films to be obtained, excellent close contact properties between electroconductive films and substrates, and the like. Used for such electroconductive compositions are metal particles such as made of gold, silver, copper, palladium, platinum, or the like, as electroconductive fillers, and electroconductive compositions including silver as electroconductive fillers are particularly frequently used. However, the electroconductive compositions adopting silver fillers are defective in that migration is apt to occur.
Concerning restriction of migration, it is effective to employ copper as a filler, which is also excellent in terms of electroconductive characteristics, cost, and the like. As a method for forming an electroconductive film by adopting copper as a filler, attention has been recently and particularly directed to a technique for realizing low-temperature calcining by adopting copper particles having an averaged particle size of 100 nm or less. For example, in JP-A-2004-256757 and JP-A-2004-327229, there is disclosed a method for forming an electroconductive circuit, by adopting an electroconductive ink containing a copper powder having particle sizes of 30 nm or less, and drawing an electroconductive pattern by an ink jet scheme, followed by heating at a temperature of 200° C. or lower in a nitrogen atmosphere.
However, it is required to repeat drawing and calcining in order to form a circuit having a film thickness at a level of about several μm by the ink jet scheme, thereby exhibiting a disadvantage of complicated process. Further, the above-described method is said to enable adoption of fine particulates of copper and obtainment of electroconductivity by low-temperature calcining by virtue of addition of a dispersant such as citrate like ammonium citrate, or a tertiary amine type monomer. However, electroconductive films obtained thereby have specific resistances which are 20 to 50 times as high as those of bulk copper, thereby failing to attain a sufficiently satisfactory level.
As means for overcoming such defects, disclosed in JP-A-7-320535 is a method for forming a thick-film electroconductive circuit having a film thickness of several μm or more by adopting a screen printing scheme, a dispenser scheme, and the like. This method is described to produce fine metal particles having an averaged particle size of 100 nm or less by adopting an organic metal salt as a precursor, and to bring the fine particles into a mixture with copper-based particles having an averaged particle size of 0.2 μm or more, thereby enabling obtainment of a low specific resistance which is two or less times that of bulk copper.
However, in the method, it is required to form the fine particles having an averaged particle size of 100 nm or less, by adopting the organic metal salt such as carboxylate as the precursor, and by once heating it. Further, the calcining condition of the electroconductive composition includes a temperature as extremely high as about 900° C. in a nitrogen atmosphere, and the sintering start temperature is also as high as about 400° C., thereby exhibiting such a defect that the method is inapplicable to a polymer substrate, for example, and is limited to usage such as formation of a circuit on a substrate like a ceramic substrate having a higher heat-resistance temperature.
In turn, disclosed in JP-A-2004-534362 as a technique for lowering a calcining temperature is a method for forming a thick-film electroconductive circuit by a composition comprising: copper fine particles having an averaged particle size of 0.3 μm or less, and copper particles having an averaged particle size of 0.3 μm or more; and a reactive compound such as a decomposable organocopper compound for depositing metal copper by thermal decomposition, or carboxylic acid capable of producing such a decomposable organocopper compound. It is also disclosed therein to bring the particles having the averaged particle size of 0.3 μm into a mixture with particles having a different averaged particle size. According to this method, it is described that the decomposable organocopper compound is decomposed to produce copper during heating, and a low resistance which is about 3 times that of bulk copper can be realized by calcining at 330° C.
However, the above method requires a specific mixed gas of nitrogen-steam-hydrogen as a calcining ambient, thereby exhibiting problems of safety, economical efficiency, and the like. Moreover, since the calcining temperature is also as high as about 330° C., the method is difficult in application to a polymer substrate such as a polyimide substrate having a heat-resistance temperature of 300° C. or lower, for example. It is thus required to attain calcining at a lower temperature, for easy application to a polymer substrate.
As described above, electroconductive compositions including copper as fillers have not reached a position of mainstream of electroconductive compositions, due to such reasons that copper essentially and internally has a problem of degradation due to oxidation, and high temperatures are required for calcining copper, and so on. As such, there is sought for an electroconductive composition that is easy in formation of electroconductive circuit, that provides an electroconductive film having a low specific resistance of about 5 μΩ·cm which is about 3 times that of bulk copper, that is not limited in kind of substrate and has an excellent close contact property with a substrate, and that is low in cost.
Incidentally, in addition to the above demand to obtain an electroconductive circuit having a low resistance by adopting an electroconductive composition including copper as filler, there is also a demand for an electroconductive composition that can be calcined in the industrially and easily achievable atmospheric air, and that is capable of providing an electroconductive film having a resistance value within an allowable range as an electronic component such as a specific resistance of about 100 μΩ·cm which is about 60 times that of bulk copper, for example.
Namely, copper is considerably deteriorated in performance due to oxidation such that calcining is necessarily conducted in an inert atmosphere or a reducing atmosphere, so that copper is limited in applicability insofar as in a present state. Thus, if there is provided an electroconductive composition capable of forming an electroconductive film having a specific resistance within a certain allowable range even by calcining in the atmospheric air, such an electroconductive composition is capable of substituting for conventional ones required to be calcined in an inert atmosphere or reducing atmosphere in many applications where such a specific resistance is not problematic.